The present invention relates to a process for recovering heat in a distillation process.
In the petroleum refining, petrochemical and oil and fat industries where feeds such as crude oil, bottom products after atmospheric distillation, fatty acids and so on are subjected to distillation to obtain various products as distillates and bottom products, it has become a very urgent problem how to recover the heat which has been hitherto wasted in cooling processes because the costs of fuels have increased tremendously and will steadily increase so that the energy savings are essential.
Referring to FIG. 1, a prior art distillation process will be described. The feed flows through a main feed pipe c and is preheated successively by heat exchangers i, h and g inserted into the side-stream product withdrawing pipes f, e and d. Thereafter the feed is further preheated when it passes through a heat exchanger k inserted into the bottom product or residue withdrawing line j, is heated by a heating furnace b and is charged into a distillation column a.
The separation of various side-stream products is effected based upon the difference in boiling temperature among them. Light components thus separated are withdrawn through the lines d, e and f and heat exchangers g, h and i, transfering their heats to the feed flowing through the main feed line c. As a result, the side-stream products are cooled to about from 130 to 230.degree. C. They are further cooled by coolers l, m and n, which use the water or air as coolant, to the temperatures at which they may be safely stored, and they are stored in storage tanks q, r and s as the final or intermediate products.
Part of the side-stream product, which is obtained from the point closest to the top of the distillation column a is returned through a return line o to the top of the distillation column a as a reflux after having been cooled by the cooler n in order to improve the separating efficiency. The bottom product flows through the pipe line j into the heat exchanger k where it transfers its heat to the feed which has been preheated to some extent when it passed through the heat exchangers i, h and g as described above. The bottom product is further cooled by a cooler p and stored in a storage tank t as the final or intermediate product. Even though not shown, low-pressure steam generators may be disposed upstream of the coolers l, m, n and p.
As described above, according to the prior art distillation process, the heats of the distillates and bottom product are used to preheat the feed, thereby attaining the savings of energy. However, as will be described below, the prior art distillation processes have some serious problems in view of the energy savings. Firstly, the feed is preheated successively by the respective higher-temperature portions of the distillates and bottom product of the lines f, e, d and j in the order, the boiling temperatures of the products of these lines being increased in the order named. As a result, the heats of these products are not recovered to a satisfactory extent. That is, in order to attain a high heat exchange efficiency between two liquids, the higher the difference in temperature between them, the better. However, the feed is preheated to a considerably high temperature by the distillate with the lowest boiling point of the line f so that when it flows into the heat exchanger h, the difference in temperature becomes less between the feed and the distillate with the next-lowest boiling temperature of the line e. The temperature difference becomes further less as the feed flows into the heat exchanger g and into the heat exchanger k. In order to keep a desired amount of heat exchanging when the temperature difference is less, the surface of the heat exchanger must be increased, but it is impossible to increase it infinitely from the technical and economical standpoints. As a result, it has been the general practice to make the temperature difference as high as possible between the feed preheated in the preceding stages and a side-stream product or bottom product with a higher boiling temperature. However, this arrangement results in the unsatisfactory heat recovery from a side-stream product with a lower boiling point. As a consequence, the temperature of the side-stream product with the lower boiling temperature is too high to feed the product directly into a storage tank, so that a cooler must be provided to cool the product further to a desired level. In addition, the product is made to flow through a low-pressure steam generator (not shown) as described elsewhere, but the pressure of the steam obtained is so low as less than 5 kg/cm.sup.2 G. As a result, the use of the steam is limited and an excessive quantity of the steam is left.
Furthermore, the heats of the side-stream products and the bottom product are taken away substantially by the coolant such as water or air so that the prior art distillation processes cannot attain the heat recovery to a satisfactory degree.
In view of the above, the primary object of the present invention is to provide a process which may recover the heats from the side-stream and bottom products which have been hitherto wastefully taken away by the cooling water or air when passed through the coolers. The present invention will become more apparent from the following description of one preferred embodiment thereof taken in conjunction with the accompanying drawings.